Oncogene-directed cancer treatments and antimicrobial therapies are often thwarted by rapid acquisition of drug resistance, a chief barrier to lasting remission. Promising insight is emerging from a seemingly distant field - protein folding. To function, proteins must adopt complex, often metastable conformations. Perilously, many diseases arise from folding or misfolding of a single protein. My previous studies have focused on Hsp90, a molecular chaperone that folds metastable proteins critical for oncogenic transformation and signaling. By influencing the fold and function of an elite cohort of regulatory proteins, this chaperone has the power to influence the evolution of new traits. I will address the following aims during my remaining mentored training and initial independent research career: (I) Determine how Hsp90 transforms genetic variation. Hsp90 strongly impacts the effects of polymorphisms in Mec1/ATR, a central player in cancer signaling, enabling responses to certain genotoxic stresses at the expense of others. Biochemically and functionally, I will examine how this affects the DNA damage response, cell cycle, and viability. Additionally, is will investigate how Hsp90 impacts the transcriptional network of Pdr8, a transcription factor that enables resistance to many drugs. Finally, I will examine how Hsp90 inhibition transforms the effects of polymorphisms in cis-regulatory elements of NDI1, to create strong resistance to oxidative stresses. (II) Investigate assimilation of Hsp90-contingent phenotypes. To investigate eventual breakthrough drug resistance I will isolate causative variation initially and after assimilation (when resistance is Hsp90-independent). High-throughput genetic techniques and next-generation sequencing will provide a mechanistic understanding of this phenomenon. (III) Identify and characterize additional factors that allow highly mutated cells to survive, proliferate, and evolve new traits. Cancer cells must sustain massive mutation loads and consequently toxic proteome destabilization. I will screen for proteins that rescue growth of highly mutated strains but do not affect unmutated parents. (IV) Identify and characterize additional protein-based mechanisms that facilitate adaptation to new environments. Using digital ribosome profiling, yeast and bacterial genetics, and biochemistry I will characterize two prions, [PSI+] and [GAR+], and determine how they affect adaptation to changing environments. The results of these studies will offer detailed insight into how Hsp90, and protein homeostasis more generally, controls signaling pathways that enable drug resistance and how these mechanisms contribute to breakthrough resistance. They will also expose an Achilles' heel common to all cancers - addiction to factors that enable maintenance of massive mutation loads.